Electron scattering prevention film and method of manufacturing the same

ABSTRACT

A FILM FOR THE PREVENTION OF SCATTERING OF ELECTRONS COMPRISING AN ELECTRODE LAYER, A CROSSED LAYER FORMED THEREON CONSISTING OF AN ELECTRODE CONSTITUENT MATERIAL AND A MATERIAL FOR THE PREVENTION OF SCATTERING OF ELECTRONS SMALLER IN ATOMIC NUMBER THAN THAT OF THE ELECTRODE CONSTITUENT MATERIAL, AND A SCATTERING PROVENTION OF SCATTERING ON WHICH IS FORMED A LAYER FOR THE PREVENTION OF SCATTERING OF ELECTRONS OF THE SECOND MATERIAL ON SAID CROSSED LAYER. A METHOD OF MANUFACTURING THE ABOVE ELECTRON SCATTERING PREVENTION FILM IS ALSO DISCLOSED.

Sept. 19, 1972 KENJIRO TAKAYANAGI EIAL 3,592,576

ELECTRON SCATTERING PREVENTION FILM AND METHOD OF MANUFACTURING THE SAME2 Sheets-Sheet 1 Filed Jan. 9, 1970 f7 ll 3] 22b /BORON M ALUMINUMAHPHOSPHOR-S BORON CARBIDE. LA/ CROSSED LAYER ALumNt/M PHOSPHORS GLASSINVENTOR S K 731m Win/ 61 Er A z.

ATTORNEYS p 1972 KENJIRO TAKAYANAGI EI' ELECTRON SCATTERING PREVENTIONFILM AND METHOD OF MANUFACTURING THE SAME 2 Sheets-Sheet 2 Filed Jan. 9,1970 7L '1 7I I CROSSEDLAYER SCATTERING THIQKNESS PREVENTION LAYER HETALBACKING LAYER 5 otkw. mdFiomzou so as INVENTOR-S Kfiwmmwa/ 7 4m 7 BY 0470140 cofi fi gf ATTORNEYS United States Fatent C Int. Cl. H61 29/28US. Cl. 117-217 7 Claims ABSTRACT OF THE DISCLOSURE A film for theprevention of scattering of electrons comprising an electrode layer, acrossed layer formed thereon consisting of an electrode constituentmaterial and a material for the prevention of scattering of electronssmaller in atomic number than that of the electrode constituentmaterial, and a scattering preventing layer and on which is formed alayer for the prevention of scattering of electrons of the secondmaterial on said crossed layer. A method of manufacturing the aboveelectron scattering prevention film is also disclosed.

This invention relates to an electron scattering and reflectionprevention film and a method manufacturing of the same, and moreparticularly to such a film for use in color television display tubes tominimize the reflection and scattering of a beam of electrons.

Generally in the post-acceleration color television tubes electron beamsemitted from three electron guns are accelerated in a high voltageelectric field and strike at a phosphor surface electron beams of highkinetic energy excite the phosphors and produce a luminous output. Atthe same time a large number of secondary electrons, reflectingelectrons and scattering electrons are generated by the impact ofelectron beams of high kinetic energy.

The scattering electrons have no high kinetic energy as do thereflecting electrons, which therefore can be removed by utilizing theenergy differences between the injected electrons and the secondaryelectrons. However the scattering electrons are accelerated by saidpost-acceleration electric field and strike at the phosphors again withhigh kinetic energy of the same intensity as the injected electrons withthe result that undesirable halos are generated surrounding the luminouspoints which are produced by normal injecting electrons. As aconsequence, the contrast of the reproduced images is reduced andadverse color contamination is effected.

So, to remove the phenomena effectively it has been proposed to sinter asingle thin layer of a material of small atomic number such as boron orcarbon deposited on the metal backing of aluminum evaporated on thephosphor layer so that the amount of scattering electrons from thephosphor surface is reduced, as seen in US. Pat. No. 2,878,411. But theaforementioned conventional single thin layer was not capable ofobtaining a desired effect enough to prevent the scattering of electronsand absorb satisfactorily the secondary electrons emitted from theshadow mask. Another drawback was that the single 3,692,576 PatentedSept. 19, 1972 layer was readily peeled oif during normal heating at atemperature of about 430 C. in the course of manufacture of the colortelevision tubes and accordingly the manufacture was very diflicult.Furthermore it was desired that the layer be thicker to fully absorb thesecondary electrons which however caused the single layer more readilyto peel otf during manufacturing. In view of these drawbacks the aboveproposed construction was not entirely practicable.

The present invention is intended to provide a film for the preventionof scattering of electrons and devoid of such drawbacks as describedwhile being easily practicable.

A primary object of the present invention is to provide a film for theprevention of scattering of electrons by the bombardment of the electronbeams.

A second object of the invention is to provide a scattering preventionfilm for electrons, which can maintain itself in a very stable conditionwithout peeling off owing to temperature variations and similar factors.

Another object of the invention is to provide an electron scatteringprevention film which can offer a color television tube which is good incontrast and has no color contamination, and be particularly adapted forthe postacceleration color television tubes.

Still another object of the invention is to provide a method ofmanufacturing an electron scattering prevention film by having a crossedlayer formed between a metal backing layer and a electron scatteringprevention layer.

Further objects and features of the invention will be understood clearlyfrom the description made with reference to the accompanying drawings,in which;

FIG. 1 is a vertical cross sectional view of a post-acceleration colortelevision tube having an electron scattering-prevention film accordingto the present invention;

FIG. 2 is an enlarged partial cross section of a screen of a tube havinga conventional film for the prevention of electron scattering;

FIG. 3 is an enlarged partial cross section of a screen of a tube havingan electron scattering-prevention film, in accordance with the presentinvention;

FIG. 4 is a diagram showing a ratio of composition of an evaporated filmvarying with the film thickness;

FIG. 5 is a vertical cross sectional diagram of an embodiment of adevice for manufacturing a film for the prevention of electronscattering in accordance with the present invention; and

FIG. 6 is a perspective view of heating means for the evaporatingmaterial.

Referring to FIG. 1, a post-acceleration type color television tube 10generally consists of a glass bulb 11 and is externally provided with adeflection yoke 12. Within neck 13 of the funnel are sealed threeelectron guns provided on a connecting base 14. The bulb 11 generallycomprises the funnel neck 13, a funnel 16 and a face place 17. On theinner surface of the face plate 17 are disposed a phosphor layer 18consisting of three color dot phosphors, i.e., of red, green and blueand a film 19 for the prevention of the electron scattering laterdescribed and formed integrally with a metal backing in accordance withthe present invention. A shadow mask 20 has apertures larger in diameterthan each diameter of dots of phosphors of the phosphor layer 18provided in response to respective dot trios spaced apart fromrespective layer 18 and film 19 and in parallel therewith.

According to one aspect of the present invention, a first electric powersource E is connected to a metal backing layer and a transparentconducting electrode such as nesa glass. A second power source E of 8.8kv. is connected to a shadow mask 20 and third power source E of 9.6 kv.is connected to an anode 21 provided on the inner walls of the funnel16. The power voltages E to E of the above power sources may preferablyhave the mutual relationships of E E E E and E E so that there may beformed an intense post-acceleration electric field between the shadowmask 20 and the phosphor layer 18 with respect to the electron beams 22emitted from the electron guns 15. Between the shadow mask 20 and theanode 21 is formed a weak negative acceleration electric field, whichpermits a major portion of the secondary electron 23 emitted from theshadow mask 20 when the electron beam 22 strikes at the shadow mask 20being absorbed at the anode 21.

Some of the secondary electrons generated in the surronndings of theapertures of the shadow mask 20 are drawn to the post-accelerationelectric field between the shadow mask 20 and the phosphor layer 18, andenter the acceleration electric field through the apertures, hence areaccelerated and strike at the phosphor layer 18 partly causingdeterioration of contrast. As the initial speeds of the secondaryelectrons correspond to about ten or more volts when they are emittedfrom the shadow mask 20, the kinetic energy thereof is almost equivalentto the energy of the post-acceleration voltage when the secondaryelectrons strike at the phosphor layer 18 and said kinetic energy issmaller than that of the electron beams 22 striking at the phosphorlayer 18 through the shadow mask 20. By preferred selection ofticknesses of the metal backing layer evaporated on the phosphor layer18 and the scattering prevention film it is possible to allow theprimary electron beams 22 only to reach the phosphor screen 18 forillumination.

Thickness of the film coated on the phosphor layer 18 which may beenough to prevent the hazard of deterioration of contrast by thesecondary electrons above described is about 5,000 A. in the conversionvalue of aluminum, where for instance the voltage of the phosphor layeris 20 kv. and that of the shadow mask is 9 kv., and about 8,000 A. wherethe voltage of the phosphor layer is 25 kv. and that of the shadow maskis 11 kv.

The electron beams of high kinetic energy passing through the shadowmask 20 and striking at the phosphors emit a large number of secondaryelectrons and scattering electrons during striking at the screen. Due tosmall initial speeds these secondary electrons will not excite norilluminate the phosphors. The scattering electrons however are theelectrons tending to disperse in diverse directions with nearly the sameenergy as of the injecting electron beams. Hence, among the scatteringelectrons, electrons 24 emitted at angles larger than a critical angle(the angle is defined by the voltages of the phosphor layer and theshadow mask and the distance between the phosphor layer and the shadowmask) will follow an arcuate path in the postacceleration electric fieldagain to strike the phosphor layer losing almost no energy. As they dothis, they cause halos surrounding the luminous points, produced by thenormal electron beams. In consequence, the contrast of image willdeteriorate and the color contamination takes place. As the scatteringelectrons 24 have nearly the same energy as the primary electron beams,the lack of contrast and the color contamination can not be eliminatedeven by coating only a thicker layer or film on the phosphor layer. Thishas been a major draw-back with the prior art type of postaccelerationcolor television tubes.

As seen in FIG. 2, there has been proposed in prior art evaporationforming an aluminum metal backing layer 30 on the phosphor layerdeposited on the inner surface of the glass face plate 17 and further onthe layer 30 sintering a layer 31 for preventing the scattering ofelectrons. The layer 31 constitutes of a material smaller in atomicnumber such as boron or carbon. This sort of composition however had theeffect that although the reflection and scattering of electrons theelectrons penetrating through the prevention layer 31 were found todisperse about the boundary between the prevention layer 31 and themetal backing layer 30 besides the electrons 32a reflected and scatteredon the surface of the layer 31 and metal backing 30, thereby producingscattered electrons 32b so that a number of the back-scattered electronsin total was not reduced to a large extent. Furthermore, as the metalbacking layer 30 and the prevention layer 31 which each have a differentheat expansion coefficient were in contact with each other with a clearboundary therebetween, the prevention layer 31 and the metal backinglayer 30 invariably peeled off from each other or from the phosphorlayer 18 by difference of internal stresses in the respective layersparticularly due to temperature variations of heating and cooling orshocks.

The film for preventing the reflection and scattering of electronsaccording to the present invention removes fully the above describeddrawbacks.

An enlarged view in FIG. 3 shows a partial cross section of the screenof the tube provided with a film for preventing the scattering ofelectrons according to the present invention. By means of evaporationlater described aluminum is applied on phosphor layer 18 to form a metalbacking layer 40 thereon, which is continuously provided with a.compound or crossed layer 41 of a mixture of aluminum and a material forpreventing the electrons scattering comprising an element having asmaller atomic number; and, further formed thereon is a prevention layer42 solely consisting of the described prevention material so as toconstitute the electrons scattering prevention film integrally with themetal backing layer 40.

The described prevention material is generally assumed to comprise anelement having an atomic number of at least less than one half of theatomic number 13 of aluminum in order to obtain the contrast ratio of 20on the condition that the ratio of emission of reflected electrons isproportional to the atomic number. Such a material is required to haveproperties suflicient to meet all the requirements in the manufacturingof television tubes. With these factors in consideration the inventorshave conducted experiments with materials such as boron, carbon, LiF,LiCO and 13 C and adopted boron carbide (B C) as the most preferredmaterial.

As shown in FIG. 4, the crossed layer 41 has the composition ratio ofaluminum progressively varying from to 0% and that of the preventionmaterial varying from 0% to 100% from the side of the metal backinglayer 40 to the side of the prevention layer 42, the mixture ratios ofboth materials having successively varying density gradients.

The thickness of each layer as described above may be provided such thatscattering electrons will be reduced in number and the secondaryelectrons from the shadow mask will not illuminate the phosphor screenwhile the primary electron beams may sufliciently penetrate through thelayers and illuminate the phosphors. Distribution of thicknessesrespectively of the metal backing layer 40, the crossed layer 41 and thescattering prevention layer 42 may be preferably presented as 3:116 to2:2:6 in a preferred ratio in the conversion value of aluminum.

In the scattering prevention film shown in FIG. 3, as the crossed layerexists between the metal backing layer and the scattering preventionlayer, the primary electron beam 2217 which has penetrated into thescattering prevention layer 42 reaches the phosphor layer 18 withoutbeing dispersed and illuminates the phosphors. In effect the preventionof scattering of electrons was markedly large and also the contrast waspromoted by about twenty percent over the conventional construction.Presence of the crossed layer will not cause peeling of the scatteringprevention layer from the metal backing layer as seen to happen inconventional cases by temperature variation.

The table below shows the contrast ratios obtained by measurement of theeffect of electron scattering prevention on the face plate having thethicknesses of the layers being respectively varied, where the totalvalue of thicknesses of the metal backing layer 40, the crossed layer41, and the layer of boron carbide (B C) is made constant at 5000 A. inthe conversion value of aluminum. Also, respective thicknesses of thelayers have been calculated on the basis of the conversion value ofaluminum.

Description is now made with respect to the method and apparatus formanufacturing the electrons scattering prevention film as shown in FIG.4 as well as in FIGS. 5 and 6.

The face plate 17 having a phosphor layer on its inner surface iscarried by a substratum holder 51 in a belljar 50, which is tightlyclosed with vacuum by an oil diffusion pump 52 before evaporating.Heating means 53 shown on an enlarged scale in FIG. 6 consists of acrucible 5S surrounded by an electrode 54 and a cathode filament 56encircling said crucible.

In the crucible 55 is provided boron carbide in powder form, 57 (B C,melting point 2450 C.), and on which is deposited solid aluminum 58 (Al,melting point 660 C.). By means of the diffusion pump 52 air isexhausted from the belljar 50 to cause a vacuum of IO- to mm. Hg. Avoltage V is provided at 7 v. and an electric current A at 80 A. Theheater 56 is heated to emit thermions, which are deflected by anelectric field produced by the electrode 54 with a voltage V of 5 kv., acurrent A of 50 ma., and the thermions are concentrically bombarded atthe materials 57 and 58 in the crucible.

Then a shutter 59 is closed and the materials 57 and 58 are heatedpreliminarily for two to three minutes. By this heating the gas in thematerials 57 and 58 will be released. Aluminum 58 will melt and aportion thereof will permeate through the boron carbide so as to form acompound of both materials.

After the preliminary heating the shutter 59 is opened and the voltagein the electrode 54 is raised slowly from 5 kv. to 8 kv. duringapproximately five minutes. At the instant the aluminum 58 which islower in melting point first evaporates to the phosphor layer of theface plate 17 and thereby the metal backing layer 40 is formed in thethickness of 1500 A.

With the voltage of 8 kv. being retained for a further five minutes thecompounded and crossed portion of said two materials will evaporate. Acrossed layer 41 will then be formed continuously retaining aluminum andboron carbide in a compounded and crossed condition of a gradient ofdensity composition ratio in a thickness of 500 A. and having noboundary with the metal backing layer 40. The high voltage current Aflowing at a voltage of 8 kv. is about 100 ma.

Continuing the heating evaporates the boron carbide 57 which hasremained after evaporating of the aluminum 58 and thus the vacuumevaporation is completed. On the crossed layer 41 is formed a scatteringprevention layer 42 of boron carbide successively in a thickness of 3000A. without having a boundary surface.

The leaked thermions from the heating means 53 and the electronsreflected after bombarding at the material to be evaporated in a largenumber will bombard at the face plate of the substratum, which is heatedor charged up and causes an electric discharge and peeling off of thelayers. Sometimes it may occur that the air contained in the evaporatingmaterial in the crucible 55 will expand by heating or the powders willdisperse by Spurting. The powders may reach the substratum andcontaminate the evaporating surface to cause peeling. Therefore it is soarranged that a meshed collector electrode 60 is provided so as to avoidthe occurrances of these phenomena. The meshed collector electrode 60 isdisposed between the crucible 55 and the face plate 17. The collectorelectrode 60 may be at a ground potential but it may have an appropriatebias potential. The electrode 60 will have an electrostatic shieldeffect absorbing the electron flow and the molecules of chargedevaporating materials and will not cause the peeling effect as describedhereinbefore.

The above embodiment used two kinds of evaporating materials 57 and 58.Three or more evaporating materials may also be used for forming theevaporated film. A plurality of evaporating materials combined intoalloy or reacting with each other during heating may also be used.

The requirements pertinent for realizing this invention may be seen fromthe result of the above experiments and are as follows. The totalthickness of the scattering prevention layer 42 of 8 C the crossed layer41 of B C and Al, and the metal backing layer 40 of A1 must be 5,000 A.in the conversion value of aluminum to obtain the contrast ratio of 20.The thickness of the scattering prevention layer 42 of B C should bemore than 2,500 A. in order to obtain the contrast ratio of 20. Thelarger the thickness the better is the effect of prevention of thescattering of electrons. The thickness of the crossed layer 41 is up to500 A., the larger is the thickness of the layer 41 the larger is theeffect of prevention of scattering. In case the thickness of the crossedlayer 41 is more than 500 A. the contrast ratio may be raised abouttwenty percent more than in case the crossed layer does not exist.

In the above embodiment, the invention was merely applied in thepost-acceleration color television tube. However this invention is notonly restricted to this example, generally it may be applied in allelectron beam devices which are required to have fewer scatteringelectrons to be generated by impact of electron beams, particularlycathode-ray beams.

What we claim is:

1. An electron scattering-prevention film for an electron beam device,said film comprising a metallic electrode layer which is penetrable byprimary electron beams, a crossed layer, and a scattering-preventionlayer, said scattering-prevention layer being of a compound of anelement having an atomic number less than one half the atomic number ofthe metal of said electrode layer, said crossed layer having acontinuously varying composition from said scattering-prevention layerto said electrode layer ranging from zero to of said metal and from 100to 0% of said compound, the total thickness of said layers beingsubstantially at least 5000 A., and the thicknesses of said electrodelayer, crossed layer, and scattering-prevention layer ranging from 311:6to 2:216 in the conversion value of aluminum.

2. An electron scattering-prevention film as claimed in claim 1, whereinsaid metal is aluminum.

3. An electron scattering-prevention film as claimed in claim 1, whereinsaid compound is selected from the group consisting of boron, carbon,lithium fluoride, lithium carbonate, and boron carbide.

4. An electron scattering-prevention film as claimed in claim 3, whereinsaid compound is boron carbide.

5. An electron scattering-prevention film as claimed in claim 4, whereinsaid metal is aluminum.

6. A method of manufacturing an electron scatteringprevention film foran electron beam device by an evaporation-deposition process, said filmcomprising a metallic electrode layer which is penetrable by primaryelectron beams, a crossed layer, and a scattering-prevention layer, saidscattering-prevention layer being of a compound of an element having anatomic number less than one half the atomic number of the metal of saidelectrode layer, said crossed layer having a continuously varyingcomposition from said scattering-prevention layer to said electrodelayer ranging from zero to 100% of said metal and from 100 to 0% of saidcompound, the total thickness of said layers being substantially atleast 5000 A., and the thicknesses of said electrode layer, crossedlayer, and scatteringprevention layer ranging from 321:6 to 2:2:6 in theconversion value of aluminum, said method comprising arranging asubstrate for evaporation-deposition of the required film; selectivelyheating a mixture of said metal and said compound, said compound beingof a higher melting point than that of said metal; applying a firststage of heat to said mixture to evaporate said metal to deposit thesame on said substrate; applying a second stage of heat to evaporate amixture of said metal and said compound to deposit said crossed layer;and continuing the application of heat to deposit said compound to formthe scattering-prevention layer.

7. A method of manufacturing an electron scatteringprevention film asclaimed in claim 6, wherein said heating and depositing steps areconducted in a single crucible of a single electron beam heating deviceso as to successively deposit said electrode layer, said crossed layerand said scattering-prevention layer, whereby owing to differeuces inthe melting points of said metal and said compound, the composition ofsaid crossed layer continuously varies.

References Cited UNITED STATES PATENTS 3,515,587 6/1970 Letter RALPH S.KENDALL, Primary Examiner US. Cl. X.R.

ll72l9, 33.5 C, 211; 313-92 R

